An actuating element may, for example, be an actuating rail or actuating rod which acts on an actuating fork or on a sliding sleeve via the actuating fork. A piston drive is actuated by a hydraulic pressure medium or by compressed air. The piston drive generally has two pistons which interact with one another. In this context, the actuating element is slidably mounted with respect to a housing of a change speed gear mechanism in a longitudinal bonded fashion, movable alternately in one direction or the other, by the pistons which can longitudinally move in pressure spaces. In each case, a piston preferably engages at an end of the actuating rail or rod which points in the longitudinal direction. During an actuating movement of the actuating element, pressure is applied to one of the pressure spaces by the pressure medium so that the piston which bounds this pressure space generates a force on the actuating rail. The force is directed longitudinally with respect to the actuating movement, and either displaces the actuating element longitudinally or supports the displacement of the actuating element. The device can therefore be used in transmissions in which the actuating element is either exclusively activated using the pressure medium or for which activation is supported by the pressure medium. The device with power-assisted activation may be used in utility vehicles in which the manual gear shift operation is servo-supported.
European Patent No. 1 164 316 A2 discloses a generic device. A piston on an actuating rail has, at its outer circumference, a sealing element between the piston and a pressure space. That space is usually of cylindrical design. A sealing element is formed from at least one sealing lip of a seal which bears, at least temporarily, against the wall. In this context, the seal seals off the pressure space from the rest of the surroundings of the transmission in a pressure-tight fashion or with a low level of leakage. The pressure space is bounded on one side by the piston and has a variable size set by the stroke of the piston. The seal guides or supports the piston against the wall of the pressure space. With respect to transverse forces, the actuating element is supported by the wall of the pressure space by the pistons via the seal or seals. The seal is usually formed from plastic or rubber-elastic materials and is disadvantageously loaded by forces acting transversely with respect to the longitudinal movement of the piston. The pressure space is usually inadequately sealed and the piston or the seal wear prematurely.
European Patent EP 1 164 316 A2 discloses guiding actuating elements by bearing units which are free of play and which are composed of roller bodies and raceways, which are prestressed in a sprung fashion against the roller bodies. In such a bearing arrangement, the actuating element is usually supported in the transmission in an at least a linearly movable fashion by ball bearings. The roller bodies roll on raceways of thin-walled inner rings or outer rings. The rings are formed from sheet steel. When the bearing is installed, at least one of the bearing rings compensates both tolerances in the receptacle of the bearing in the housing of the transmission. These tolerances are caused by production and alignment errors of the centers of pressure spaces, which are opposite one another. The bearing ring also prestresses the roller bodies in the bearing without play. The actuating element is precisely guided by the bearing units in a manner that is free of play and low in friction. Previously, such a bearing arrangement has been considered disadvantageous owing to the considerable expenditure for equipment. For this reason, a guide section is positioned in front of the piston. The guide section guides the piston in the pressure space and supports the actuating rail in the pressure space. A lining of the pressure space is therefore not required. The cylinder bore which guides the piston is introduced directly into the lightweight metal housing of the transmission. This solution disadvantageously produces frictional forces at the longitudinally moved guide section, in addition to producing frictional forces arising from the moved seal. Alternating forces, which arise from uneven or excessively high friction, prevent a constant pressure buildup. This causes constant sliding at the actuating element and a constant actuation force at the sliding sleeve. This impedes precise control of the actuation in a manner which is dependent on the displacement or force/displacement. The pressure spaces which are formed in opposite walls of the transmission housing have to be aligned precisely with one another. Support deviation of the centers of the bores with respect to one another cause the guide sections to tilt and thus lead to higher frictional forces in the device. Furthermore, introduction of the bores for the pressure spaces into the walls of the transmission housing entails relatively high cost.